I was flying back on a partner airline connecting to UA and wanted to change my seat on the UA flight (odd row on Polaris opened up), but the UA app/site wouldn't let me since it was within 24hrs of the first segment. The UA app/site kept telling me I needed to check-in to change my seats and UA wouldn't let me check-in since the first segment was on partner airline....
Is there any reason they lock the seat selection 24hrs before the flight unless you are checked in?
Most airline softwares transition control of the flight from the Reservation System to the Departure Control System 24 hours prior to the flight. In the case of UA, SHARES has both DCS and Reservation System functionality, but I'm guessing that it's two separate modules of the software.